This article is more than 5 years old. Information may no longer be current.
Imaging device could assist in determining refractive surgery’s impact on vision
Functional magnetic resonance imaging was used to investigate LASIK's impact on visual function beyond the posterior eye segment.
Many researchers have been studying refractive surgery’s influence on visual function and the evaluation method of refractive surgery. However, they have been investigating the influence and the evaluation method with respect to the anterior and posterior eye segments. We have investigated the influence of refractive surgery on the visual pathway beyond the posterior eye segment.
The evaluation of the influence of refractive surgery on the visual cortex was studied as a cerebral functional appraisal method using functional magnetic resonance imaging (fMRI).
Study
The study participants were seven healthy female volunteers who received LASIK. In all participants, visual acuity without correction for distance and refractive error were examined. No cases were excluded due to surgical or postoperative complications. All of the participants were right-handed and had no history of ophthalmologic, neurological or psychiatric illness. Their ages ranged from 24 to 34 years.
An experimental procedure using fMRI was performed on a 1.5 Tesla GE Signa (General Electric) using a standard quadrature head coil (TR: 3,000 m; TE: 60 ms; FOV: 32 cm; matrix: 64 × 64; thickness: 5 mm).
Visual stimuli were back-projected onto a translucent screen placed at the participants’ feet. Participants viewed the visual stimuli using the head coil’s built-in mirror. Stimuli were created using in-house software. The stimulation period of 24 seconds consisted of the presentation of horizontal and vertical sine wave gratings of 15 minutes in a circle of 9°, flickering at a frequency of 8 Hz. This phase was set to activation phase. The control stimuli were a 50% gray circle of 9°, with the fixation point at the center. This phase was set to control phase. Both phases were alternated five times during the sequence. Data analysis was performed with SPM99 (Wellcome Department of Cognitive Neurology) and Brain Voyager QX (Brain Innovation).
Each participant was examined four times: preoperatively and at 1 week, 3 months and 1 year postoperatively. Preoperatively, each participant wore glasses, which were targeted for emmetropia.
At 1 week, 3 months and 1 year postoperatively, stimulation was performed without correction.
Results
Preoperatively with glasses for emmetropia, participants showed remarkably consistent activations detected bilaterally in the primary visual cortex.
Postoperatively without glasses, remarkably consistent activations were detected bilaterally in the primary visual cortex. In addition, the range of activation seen was larger than that observed preoperatively.
At the 3-month period without glasses, remarkably consistent activations were detected bilaterally in the primary visual cortex. The range showing activation was the same as that observed preoperatively.
At 1 year without glasses, remarkably consistent activations were detected bilaterally in the primary visual cortex. The range showing activation was the same as that observed preoperatively.
For the amplitude of vascular response, peristimulus time histogram in the primary visual cortex, the response at 1 week postoperatively was higher than preoperatively or at 3 months or 1 year postoperatively.
Discussion
In the last 10 years or more, fMRI has come to be used as a method of evaluating cerebral function. This method is based on the measurement of brain activity induced by regional cerebral blood flow changes. This method catches hemodynamic changes in the amount of blood flow, accompanied by brain activity, partly from a phenomenon of magnetic resonance.
In connection with activity of the cerebral cortex, the amount or speed of inflow of blood to a peripheral blood vessel floor increases. Because activity of the cerebral cortex is anaerobic, the ratio of deoxyhemoglobin/oxyhemoglobin falls relative to the blood vessel floor. Because deoxyhemoglobin is a magnetic substance in the living body, it creates phenomena in the magnetic resonance signal, causing a detectable rise in signal.
Because we examined participants who received LASIK, we used our original stimulation. This stimulation entailed presenting simultaneous vertical and horizontal lines of the same quantity. As a result of this stimulation, the amplitude of the vascular response of the primary visual cortex was higher at 1 week postoperatively than preoperatively or at 3 months or 1 year postoperatively.
Our results suggest that several months may be required for the neural basis of adaptation to new vision after refractive surgery, and that fMRI may help explore the influence of refractive surgery on vision.
For more information:
Takuya Shiba, MD, can be reached at the Department of Ophthalmology, The Jikei University School of Medicine, Tokyo, Japan; email: shiba@jikei.ac.jp.
Disclosure: Dr. Shiba has no relevant financial disclosures.